1. Field of the Invention
The present invention relates to an apparatus and method for predicting wave propagation characteristic of a multiple antenna system; and, more particularly, to an apparatus and method for predicting wave propagation characteristic of a system, in which electric wave is sent by multiple antennas and received by a single antenna in an urban canyon model and so on, by searching for all propagation paths of multiple reflected waves from respective image antennas.
This work was supported by the IT R&D program of MIC/IITA [2005-S-046-03, “Development of the basic spectrum resource utilizing technology”].
2. Description of Related Art
A variety of mobile communication systems, such as Personal Communication System (PCS), WiBro, Wireless Local Area Network (WLAN) and so on, have been established and are being currently serviced. The use of such wireless communication equipments has mainly been concentrated in urban areas. Therefore, it is a tendency that frequency is increased and a multiple antenna system (Multiple Input Multiple Output: MIMO) is introduced for increase in capacity.
It is expected that multiple antennas will be introduced for capacity expansion in most base stations in the future and terminals will have a single antenna owing to problems with implementation. Hence, a precise prediction of received power in such multiple antenna system is necessary to properly determine a position of a base station antenna and a service area of microcell.
In addition, in order to find out wave propagation characteristic of microcell, an appropriate urban modeling is required. To this end, a canyon model consisting of three loss dielectrics was generally introduced to predict a received power. In such a canyon model, a transmitting antenna and a receiving antenna are arranged orthogonally to the ground, and thus, numerous reflected waves exist therein. At this time, if a propagation path, through which an electric wave originating from the transmitting antenna arrives at the receiving antenna, is known, it is possible to obtain a reflection coefficient at each reflection point and also to know how many times reflection occurs among propagation paths of reflected waves. For this purpose, an image technique has been introduced.
FIG. 1 is an explanatory diagram showing one example of coordinate of a general urban canyon model, FIG. 2A is a view showing one example of a general generation process of image antennas, and FIG. 2B is an explanatory diagram showing one example of giving number to image antennas according to the prior art.
As illustrates in FIG. 1, the prior art method models a straight road in urban areas in dielectric canyon composed of a left building #1 1, a right building #2 2, and a road (the ground) 3.
Here, ε1 denotes permittivity for determining the material of the left building #1 1, ε2 represents permittivity for determining the material of the right building #2 2, and ε3 indicates permittivity for determining the material of the ground 3.
In addition, μ1 denotes permeability for determining the material of the left building #1 1, μ2 indicates permeability for determining the material of the right building #2 2, μ3 represents permeability for determining the material of the ground 3.
Further, a transmitting antenna 4 and a receiving antenna 5 exist within the urban canyon.
Here, the transmitting antenna 4 consists of four multiple antennas and its coordinate represents a representative value, that is, (xt, yt, zt). And, the receiving antenna 5 has a coordinate of (xr, yr, zr).
Meanwhile, an electric wave originating from the transmitting antenna 4 propagates in every direction. The wave includes a direct wave arriving directly at the receiving antenna 5, and multiple reflected waves that suffer from one or more reflections from three surfaces, that is, two building wall surfaces (left building #1 1 and right building #2 2) and the ground 3 in the urban canyon, and then arrive at the receiving antenna 5.
With regard to these multiple reflected waves, an image technique is introduced to precisely find which position reflection occurs among each building wall surface (left building #1 1 and right building #2 2), and the ground 3.
First, it is assumed that two building wall surfaces (left building #1 1 and right building #2 2) infinitely extend in y- and z-axis directions and the ground 3 infinitely extends in y-axis direction as well such that the size of each reflection surface is much greater than the wavelength of electric wave used.
By this assumption, there are numerous image antennas generated on two building wall surfaces (left building #1 1 and right building #2 2) and image antennas further generated below the ground by the generated image antennas and the transmitting antenna 4 on the ground.
In this case, a received power by the direct wave and multiple reflected waves received by the receiving antenna 5 can be expressed as follows:
                              P          R                =                                                            P                T                            (                              λ                                  4                  ⁢                  π                                            )                        2                    ⁢                                                                                    ∑                                      n                    =                    0                                    ∞                                ⁢                                                      G                    n                                    ⁢                                      R                    n                                    ⁢                                                            ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                                                  kr                          n                                                                                                            r                      n                                                                                                          2                                              Eq        .                                  ⁢                  (          1          )                    
wherein PR denotes a received power by a direct wave and multiple reflected waves received by a receiving antenna, PT denotes a transmitted power, λ represents a wavelength of electric wave, k indicates a wave number, and n denotes a wave propagation path number where, if n is 0, this represents a direct wave and other values all represent reflected waves. Further, Gn denotes a square root of gain product of transmitting and receiving antennas lying on an n-th wave propagation path, which depends on the relative position between the transmitting and receiving antennas 4 and 5 when the directivity of antennas is considered. Also, Rn denotes a path reflection coefficient which is obtained by multiplying a reflection coefficient of each reflected wave reflected from the building wall surface (left building #1 1, right building #2 2), or the ground 3) on each propagation path by the number of times of reflection, and rn represents a distance of wave propagation path between n-th receiving image antennas.
The following is a description for a conventional algorithm of searching for propagation paths of a direct wave and multiple reflected waves that exist within an urban canyon model by using an image technique.
When an electric wave sent from the transmitting antenna 4 is reflected from two dielectric surfaces 1 and 2 which are the building wall surfaces of FIG. 1, an infinite number of image antennas corresponding to reflected waves, that is, reflected waves from wall surfaces are generated. Thus, image antennas are also generated below the ground, which correspond to reflected waves including reflected wave once from the ground.
In this regard, an infinite number of image antennas (designated by rnv) generated by two building wall surfaces 1 and 2 and the ground 3 will be described below. Here, n denotes a number of each image antenna relative to the building wall surfaces (left building #1 1 and right building #2 2) and v indicates a number of an image antenna relative to the ground 3. At this time, a number of an image antenna on the ground is given 0 and a number of an image antenna below the ground is given 1. Therefore, an image antenna on the ground is represented by Rn0 and an image antenna below the ground is designated by Rn1.
First, image antennas by two building wall surfaces (left building #1 1 and right building #2 2) are treated, and thereafter, image antennas by the ground 3 is treated. In particular, an infinite number of image antennas generated by two building wall surfaces (left building #1 1 and right building #2 2) are numbered as follows.
The actual receiving antenna 5 is indicated by R00 by giving n=0 thereto. Further, as for image antennas by reflection by two building wall surfaces (left building #1 1 and right building #2 2), as shown in FIG. 2A, odd numbers are given sequentially to image antennas that exist in regions where an x coordinate is less than 0 (i.e., x<0), and even numbers are given sequentially to image antennas that fall in regions where an x coordinate is larger than 0 (i.e., x>0). This numbering rule may be expressed, as in rectangular wave shown in FIG. 2B.
First, the actual antenna generates R10 and R20 which are two image antennas. Next, the following image antennas consecutively generated from R10 have lower limit numbers of the rectangular wave and the following image antennas consecutively generated from R20 have upper limit numbers of the rectangular wave.
At this time, in case of calculating a received power by Eq. (1), when an electric wave from each image antenna arrives at the receiving antenna 5, it is required to know that reflections occur several times by the left building #1 1 and the right building #2 2.
With respect to odd image antennas R10, R30, R50, R70, etc., initial reflection actually occurs in the left building #1 1 starting from the transmitting antenna 4, and arrives at the receiving antenna 5 via each of the remaining paths.
On the other hand, with respect to even image antennas R20, R40, R60, R80, etc., initial reflection actually occurs in the right building #2 2 starting from the transmitting antenna 4, and arrives at the receiving antenna 5 via each of the remaining paths.
In rectangular wave of FIG. 2B, pairs of antennas that lie in vertically same positions, that is, {0}, {1,2}, {3,4}, {5,6}, etc. have the number of times of reflection (mn=0, 1, 2, 3, etc.) in sequence from the front part. A total number of times of reflection of each image antenna for an image antenna number n, mn, is represented as follows:
                              m          n                =                                            (                                                2                  ⁢                  n                                +                1                            )                        +                                          (                                  -                  1                                )                                            n                +                1                                              4                                    Eq        .                                  ⁢                  (          2          )                    
wherein mn indicates a total number of times of reflection of each image antenna for an image antenna number n, and n=0, 1, 2, 3, etc.
At this time, a reflection process of an image antenna below the ground is identical to that of an image antenna on the ground except that it includes ground reflection once more.
Meanwhile, a coordinate (xn, yn, zn) of an (n, v)-th image antenna can be expressed, from Eqs. (1) and (2), as follows:
                                          x            n                    =                                                                      (                                      -                    1                                    )                                                  m                  n                                            ⁢                              x                t                                      +                                          {                                                                                                    (                                                  -                          1                                                )                                            n                                        ⁢                                          m                      n                                                        +                                                            1                      +                                                                        (                                                      -                            1                                                    )                                                                                                      m                            n                                                    +                          1                                                                                      2                                                  }                            ⁢              w                                      ⁢                                  ⁢                              y            n                    =                                                    y                t                            ⁢                                                          ⁢                              z                r                                      =                                                            (                                      -                    1                                    )                                v                            ⁢                              z                t                                                                        Eq        .                                  ⁢                  (          3          )                    
wherein mn denotes a total number of times of reflection of each image antenna for an image antenna number n, and w indicates a width of road.
As described above, the sequence of generation of image antennas and the rule of their position can be found out by giving a number to each image antenna using the numbering technique of rectangular pulse shape and searching for a position coordinate.
Further, the rule can also be searched of how many times reflection occurs on two building wall surfaces for each image antenna number. A received power calculation formula used in free space can be introduced, in which the whole space is replaced by the free space by flying image antennas corresponding to numerous multiple reflected waves.
Especially, this numbering technique has an advantage that can search for numerous wave propagation paths. At this time, it is assumed that only vertical components in electric field exist to the ground because of the long distance between the transmitting and receiving antennas. Therefore, with respect to reflection in the urban canyon model, horizontal polarization occurs against the ground and vertical polarization occurs against two building wall surfaces. And, the gain of the transmitting and receiving antennas is fixed to 1.64 when a dipole antenna is used therein.
There are prior arts as follows: U.S. Pat. No. 6,341,223 (issued Jan. 22, 2002), entitled “Radio wave propagation prediction method using urban canyon model”, Korean Patent Laid-open Publication No. 1999-0080905 (issued Nov. 15, 1999), entitled “Method for predicting propagation characteristic of wave in consideration of polarization effects in urban canyon model”, and so on.
These prior art methods cannot recognize the effects of a received power caused by polarization directions of multiple transmitting and receiving antennas in actual urban environments.
In other words, the above-mentioned prior art methods are limited only to a case where each of the transmitting and receiving antennas is a dipole antenna, and thus, cannot recognize the effects of wave propagation characteristic when multiple transmitting antennas are used in actual urban areas.